Peto’s Paradox and Evolutionary Suicide

Humans gets cancer with high frequency, while larger mammals, like whales, don’t. If cancer is essentially a negative outcome lottery at the cell level, and larger organisms have more cells, and thus more potentially cancerous cell divisions, you would expect larger organisms to be more predisposed to cancer.

Cell transformation is an extremely rare event because it requires the coincidence in a single cell of several very improbable events. However, the life-span risk of human cancer is high (ca. 20%) because of a huge number of cells in the body (ca. 1014) and large longevity. Indeed, no matter how unlikely is the event by itself it has a real chance to occur under such conditions. If so, one might suggest that animals with a small body weight and short life-span (e.g., rodents) should not suffer from cancer at all, while big animals (whales) should get cancer in their mothers’ wombs. Reality, however, does not follow this theory’s predictions (the well known Peto paradox). All animals regardless of body weight and longevity suffer from cancer, but, on the other hand, cancer incidence does not threaten the species existence.

The absence of evolutionary suicide — an adaptation that results in an entire population becoming extinct — in some populations, like large mammals, is an intriguing puzzle. There have been many explanations put forward, some of which seem plausible (e.g. Nagy, et al. (2007)), but it is worth pondering in more contexts than cancer.